The present invention relates to an emission control system for diesel and other lean-burn engines and, more particularly to an improved reductant storage system. Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert regulated exhaust constituents such as carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx), produced during the operation of the engine, into unregulated exhaust gas.
Vehicles equipped with diesel and lean-burn gasoline engines can offer the benefit of increased fuel economy, however, the control of NOx emissions in such systems is challenging due to the high content of oxygen in the exhaust gas. Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously reduced to nitrogen (N2) over a catalyst typically composed of base metals through active injection of a reductant, such as ammonia rich urea, into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency.
In the systems described thus far, the ammonia rich urea is an aqueous solution which is stored in an aqueous urea reservoir in the vehicle and is delivered to an injector, located in the exhaust system of the vehicle, via fluid lines and a pump. The urea typically has a freezing temperature in the range of 12 degrees Fahrenheit, which may present winter challenges to the operation of the SCR system. Systems which include heating elements in a submersible pump module within the reservoir have been found to operate in a less than satisfactory manner as have resistor grid heating mats within the reservoir due to poor heat transfer throughout the fluid.